Hyponatremia is the most common electrolyte disorder. As it may be asymptomatic, seizures may lead to serious symptoms such as coma. Hypothyroidism is one of the rare causes of hyponatremia. In hypothyroidism; it is thought that the increase in ADH as well as the decrease in renal plasma flow and glomerular filtration rate (GFR) cause the disorder in dilution capacity. This case was presented with the aim of reminding that hypothyroidism may be a rare cause of hyponatremia and inappropriate ADH secretion.

Material and methods:

A 15-month-old male patient with Down syndrome was admitted to the pediatric nephrology clinic for the diagnosis of hyponatremia before planning surgery due to ventricular septal defect. He was fatigue during last two days.Levothyroxine treatment were prescribe for congenital hypothyroidism in the neonatal period but his mother discontinued the medicine at the follow-up. The patients blood pressure normal, he were confused, and turgor tonus was decreased. Laboratory tests revealed prerenal azotemia (90 mg / dL, creatinine: 0.9 mg / dL, eGFH 65 mL / min / 1.73 m2), hyponatremia (112 mmol / L) and hypothyroidism (TSH 16 uIU / mL, fT4 0,94 ng / dL) and Fractional sodium excretion (FeNa 0.1%).

Hyponatremia associated with hypothyroidism in the patient suggests an inappropriate ADH syndrome, primarily due to the suppressive effect of thyroid hormones on ADH. However, the low sodium excretion in the patient, diverted to the diagnosis of hypervolemia and hyponatremia due to congestive heart failure secondary to the large VSD present in the patient was established.In this case the glomerular filtration rate also decreased due to congestive heart failure.